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I have been studying wildlife my entire life and as simple as it may sound, I have honestly come to believe (rather, let's say wonder if it is true) they migrate because they can; i.e. let's take a trip and enjoy the northern Summer.
That's what we do; why not them too? In fact, it is easier for them because they are not locked into the demands of capitalism . And they do not own things. If they want to head north, the only limiting factor is intolerably low temperatures, so it seems that if you are a healthy bird...northern Summers in the northern hemisphere are very attractive. In the southern hemisphere it would be the opposite…..right?